Production and use of hexafluoroethane

ABSTRACT

A process for production of high-purity hexafluoroethane, wherein a mixed gas containing hexafluoroethane and chlorotrifluoromethane is reacted with hydrogen fluoride in a gas phase in the presence of a fluorination catalyst at 200-450° C., for fluorination of the chlorotrifluoromethane, or wherein pentafluoroethane containing chlorine compounds with 1-3 carbon atoms is reacted with hydrogen in a gas phase in the presence of a hydrogenation catalyst at 150-400° C., and the product is then reacted with fluorine in a gas phase in the presence of a diluent gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national stage of PCT/JP02/08034 filed Aug. 6,2002 claiming benefit of U.S. Provisional Application Nos. 60/314,309and 60/314,310, both filed Aug. 24, 2001.

TECHNICAL FIELD

The present invention relates to production and use of hexafluoroethane.

BACKGROUND ART

Hexafluoroethane (CF₃CF₃) is used, for example, as a cleaning gas oretching gas for semiconductors. Several processes have conventionallybeen known for the production of CF₃CF₃, such as

(1) electrolytic fluorination processes using ethane and/or ethylene asthe starting material,

(2) thermal decomposition processes involving thermal decomposition oftetrafluoroethylene,

(3) processes in which acetylene, ethylene and/or ethane are fluorinatedusing metal fluorides,

(4) processes in which dichlorotetrafluoroethane orchloropentafluoroethane is fluorinated using hydrogen fluoride in thepresence of a fluorination catalyst, and

(5) direct fluorination processes involving reaction oftetrafluoroethane and/or pentafluoroethane with fluorine.

However, when the process of (4) above is used, for example, theresulting CF₃CF₃ contains impurities in the form of compounds derivedfrom the starting material or compounds newly produced by the reaction.Particularly problematic among such impurities are chlorine compounds,which are difficult to separate from CF₃CF₃.

When the process of (5) above is used, for example, the resulting CF₃CF₃also contains impurities in the form of compounds derived from thestarting material or compounds newly produced by the reaction. Theseimpurities also include chlorine compounds which are difficult toseparate from CF₃CF₃. Purification may be carried out before reactionwith fluorine gas in order to reduce the chlorine compounds in thestarting material, but industrial application of conventionally knownpurification processes is difficult in most cases.

The chlorine compounds in CF₃CF₃ include compounds such aschloromethane, chlorodifluoromethane, chlorotrifluoromethane,chloropentafluoroethane, dichlorotetrafluoroethane,chlorotetrafluoroethane, chlorotrifluoroethane, chlorotrifluoroethyleneand the like.

Among these chlorine compounds, chlorotrifluoromethane is difficult toseparate because it forms an azeotropic mixture with CF₃CF₃. Examples ofmethods of purifying CF₃CF₃ containing chlorotrifluoromethane includethe method described in U.S. Pat. No. 5,523,499 in which CF₃CF₃containing impurities such as trifluoromethane (CHF₃) andchlorotrifluoromethane (CClF₃) is contacted with an adsorbing agent suchas active carbon or a molecular sieve to adsorb and remove theimpurities.

Purification methods using such adsorbing agents require specialequipment for regeneration of the adsorbing agents at roughly consistentintervals in the case of constant operation, although this depends onthe impurity content. For example, in a process wherein two adsorptiontowers are provided and operation alternately switches between a step ofadsorption of impurities and a step of regeneration of the adsorbingagent, large amounts of gas may be continuously treated, but theadsorbed and removed chlorotrifluoromethane cannot be directly releasedinto the atmosphere since it is a specific freon implicated indestruction of the ozone layer, and therefore some means must be used torender it harmless.

On the other hand, pentafluoroethane (CF₃CHF₂) is used, for example, asa low-temperature refrigerant, or as a starting material for productionof hexafluoroethane (CF₃CF₃). As examples of conventionally knownprocesses for production of pentafluoroethane there may be mentioned thefollowing:

(1) a process in which perchloroethylene (CCl₂═CCl₂) or its fluorinatedproduct is fluorinated with hydrogen fluoride (Japanese UnexaminedPatent Publication No. 8-268932, Japanese Unexamined Patent PublicationNo. 9-511515),

(2) a process in which chloropentafluoroethane (CClF₂CF₃) is subjectedto hydrogenolysis (Japanese Patent No. 2540409), and

(3) a process in which fluorine gas is reacted with halogen-containingethylene (Japanese Unexamined Patent Publication No. 1-38034).

When such processes are used, the major impurities in the targetproduct, pentafluoroethane, are chlorine compounds which of coursecontain chlorine atoms in the molecules. Examples of such chlorinecompounds include compounds with one carbon atom such as chloromethane,chlorodifluoromethane and chlorotrifluoromethane, compounds with twocarbon atoms such as chloropentafluoroethane, dichlorotetrafluoroethane,chlorotetrafluoroethane and chlorotrifluoroethane, and unsaturatedcompounds such as chlorotrifluoroethylene.

When hexafluoroethane is produced by direct fluorination reaction withpentafluoroethane and fluorine gas (F₂), any of these chlorine compoundsincluded in the pentafluoroethane will react with the fluorine gas,producing chlorine, hydrogen chloride, chlorine fluorides, or variouschlorofluorocarbons. While perfluorocarbons (PFCs) included in thepentafluoroethane do not present any notable problem, chloromethane(CH₃Cl) and chlorodifluoromethane (CHClF₂) will react with fluorine gasto produce chlorotrifluoromethane (CClF₃). Hexafluoroethane andchlorotrifluoromethane form an azeotropic mixture, and therefore removalof chlorotrifluoromethane is difficult even by distillation oradsorption purification. Thus, when pentafluoroethane and fluorine gasare reacted to produce hexafluoroethane, it is preferred to usepentafluoroethane with a minimal chlorine compound content.

With conventional processes for production of pentafluoroethane, thechlorine compound content in the pentafluoroethane can be as high asabout 1 vol % in total. Repeated distillation has therefore beenconsidered to remove the chlorine compounds in the pentafluoroethane andthus increase its purity, but because of the increased production costof the distillation, as well as distillation loss, this procedure hasbeen uneconomical, while some of the chlorine compounds also formazeotropic mixtures or azeotrope-like mixtures with pentafluoroethaneand create a situation in which it very difficult to separate thechlorine compounds by distillation procedures alone. In particular,chloropentafluoroethane (CClF₂CF₃) is usually included inpentafluoroethane at a concentration of a few thousand ppm or greater,but since pentafluoroethane and chloropentafluoroethane form anazeotropic mixture, they are difficult to separate by distillation,which is the commonly employed separation and purification method.

DISCLOSURE OF THE INVENTION

It is an object of the present invention, which was accomplished underthe conditions described above, to provide an industrially advantageousprocess for production of high-purity hexafluoroethane, which can beused as an etching gas or cleaning gas for semiconductor devicemanufacturing steps, as well as the use of high-purity hexafluoroethaneobtained by the process.

As a result of diligent research in light of the circumstances of theprior art as described above, the present inventors have completed thepresent invention upon finding that the above-mentioned problems can besolved by employing a step in which a mixed gas containinghexafluoroethane and chlorotrifluoromethane is reacted with hydrogenfluoride in a gas phase in the presence of a fluorination catalyst andat a prescribed temperature, for fluorination of thechlorotrifluoromethane.

The invention therefore provides a process for production ofhexafluoroethane which comprises a step in which a mixed gas containinghexafluoroethane and chlorotrifluoromethane is reacted with hydrogenfluoride in a gas phase in the presence of a fluorination catalyst at200-450° C., for fluorination of the chlorotrifluoromethane.

The invention further provides a process for production ofhexafluoroethane comprising the following steps (1) and (2):

(1) A step in which a mixed gas containing hexafluoroethane andchlorotrifluoromethane is reacted with hydrogen fluoride in a gas phasein the presence of a fluorination catalyst at 200-450° C. to fluorinatethe chlorotrifluoromethane to convert it to tetrafluoromethane.

(2) A step in which the mixed gas containing hexafluoroethane andtetrafluoromethane obtained in step (1) is distilled to obtain purifiedhexafluoroethane.

The invention still further provides a hexafluoroethane productcontaining hexafluoroethane at a purity of 99.9997 vol % or greater,obtained using the aforementioned process.

The invention still further provides an etching gas containing theaforementioned hexafluoroethane product.

The invention still further provides a cleaning gas containing theaforementioned hexafluoroethane product.

As a result of more detailed investigation, the present inventors haveachieved completed the present invention upon finding that theabove-mentioned problems can also be solved by employing a productionprocess comprising a step (1) in which pentafluoroethane containingchlorine compounds with 1-3 carbon atoms is reacted with hydrogen in agas phase in the presence of a hydrogenation catalyst at a prescribedtemperature, for hydrogenation of the chlorine compounds, and a step (2)in which the product of step (1) is reacted with fluorine in a gas phasein the presence of a diluent gas to produce hexafluoroethane.

The invention therefore provides a process for production ofhexafluoroethane which comprises a step (1) in which pentafluoroethanecontaining chlorine compounds with 1-3 carbon atoms is reacted withhydrogen in a gas phase in the presence of a hydrogenation catalyst at150-400° C., for hydrogenation of the chlorine compounds, and a step (2)in which the product of step (1) is reacted with fluorine in a gas phasein the presence of a diluent gas to produce hexafluoroethane.

The invention further provides a hexafluoroethane product containinghexafluoroethane at a purity of 99.9997 vol % or greater, obtained usingthe aforementioned process.

The invention still further provides an etching gas containing theaforementioned hexafluoroethane product.

The invention still further provides a cleaning gas containing theaforementioned hexafluoroethane product.

BEST MODE FOR CARRYING OUT THE INVENTION

A hexafluoroethane production process according to a first aspect of theinvention will now be explained in detail.

As mentioned above, various processes are conventionally known forproduction of hexafluoroethane. As industrially safe and economicalprocesses there may be mentioned the following:

(1) processes in which dichlorotetrafluoroethane orchloropentafluoroethane is fluorinated using hydrogen fluoride in thepresence of a fluorination catalyst;

(2) processes involving reaction of tetrafluoroethane and/orpentafluoroethane with fluorine gas.

In these processes (1) and (2), compounds such asdichlorotetrafluoroethane, chloropentafluoroethane or pentafluoroethanewhich are used as the starting materials can be produced using, forexample, tetrachloroethylene (CCl₂═CCl₂) as the starting material, whilecompounds such as tetrafluoroethane can be produced usingtrichloroethylene (CHCl═CCl₂) as the starting material. Regardless ofthe method used, however, the resulting hexafluoroethane containsstarting material-derived chlorine compounds as impurities, and thecontent of impurities tends to increase at a higher reactiontemperature.

For example, pentafluoroethane (CF₃CHF₂), which is commerciallyavailable as a refrigerant, contains these chlorine compounds asimpurities. When chlorine compound-containing pentafluoroethane andfluorine gas are directly reacted for fluorination to produce CF₃CF₃,reaction between the chlorine compounds in the pentafluoroethane and thefluorine gas produces, among other compounds, chlorine, hydrogenchloride, chlorine fluorides, and various chlorofluorocarbons. Forexample, chlorodifluoromethane included as an impurity reacts withfluorine gas to produce chlorotrifluoromethane. Thechlorotrifluoromethane forms an azeotropic mixture with CF₃CF₃, and istherefore a very difficult compound to separate, even by distillation.

The hexafluoroethane production process according to the first aspect ofthe invention comprises a step in which a mixed gas containingchlorotrifluoromethane and hexafluoroethane, which are extremelydifficult to separate, is reacted with hydrogen fluoride in a gas phasein the presence of a fluorination catalyst at 200-450° C., forfluorination of the chlorotrifluoromethane. The mixed gas containshexafluoroethane at 99.9 vol % or greater, while also containingchlorotrifluoromethane as an impurity.

The fluorination catalyst used in this process is preferably a catalystwith trivalent chromium oxide as the major component, and containsnickel, zinc, indium and/or gallium in an atomic ratio of 0.01-0.6 withrespect to chromium. The catalyst may be in the form of a supportedcatalyst or a bulk catalyst, and when it is in the form of a supportedcatalyst, the carrier is preferably active carbon, alumina, partiallyfluorinated alumina or the like, with a component-supporting rate ofpreferably no greater than 30 wt %.

The temperature for the reaction between the hydrogen fluoride and themixed gas containing hexafluoroethane and chlorotrifluoromethane in thepresence of the fluorination catalyst is 200-450° C., and preferably250-400° C. If the reaction temperature is below 200° C., thechlorotrifluoromethane is not readily fluorinated, and if the reactiontemperature is above 450° C. the catalyst life is shortened, and theimpurities will tend to increase.

In the fluorination reaction, the molar ratio of the hydrogen fluorideand the mixed gas (hydrogen fluoride/mixed gas containinghexafluoroethane and chlorotrifluoromethane) is preferably in the rangeof 0.05-10, and more preferably in the range of 0.05-5. If the molarratio of the hydrogen fluoride and the mixed gas is smaller than 0.05,various chlorofluorocarbons will tend to be readily produced byside-reactions, and if it is greater than 10, an economicallydisadvantageous situation will result, such as increased reactor sizeand recovery of unreacted hydrogen fluoride.

The concentration of the chlorotrifluoromethane in the mixed gas ispreferably no greater than 0.1 vol % and more preferably no greater than0.05 vol % of the mixed gas.

The mixed gas is preferably obtained by reactingdichlorotetrafluoroethane and/or chloropentafluoroethane with hydrogenfluoride in a gas phase in the presence of a fluorination catalyst.Alternatively, the mixed gas may be obtained by reaction oftetrafluoroethane and/or pentafluoroethane with fluorine gas.

The process for production of hexafluoroethane according to the firstaspect of the invention may comprise the following steps (1) and (2).

(1) A step in which a mixed gas containing hexafluoroethane andchlorotrifluoromethane is reacted with hydrogen fluoride in a gas phasein the presence of a fluorination catalyst at 200-450° C. to fluorinatethe chlorotrifluoromethane and convert it to tetrafluoromethane.

(2) A step in which the mixed gas containing hexafluoroethane andtetrafluoromethane obtained in step (1) is distilled to obtain purifiedhexafluoroethane.

The chlorotrifluoromethane reacts with hydrogen fluoride in the presenceof a fluorination catalyst to produce tetrafluoromethane (CF₄), as shownin the following formula (1).CClF₃+HF→CF₄+HCl  (1)

Instead of reaction of the hexafluoroethane, the chlorotrifluoromethaneimpurity reacts with the hydrogen fluoride. Since the difference inboiling points between the resulting CF₄ and the unreacted residualCF₃CF₃ is about 50° C., and the CF₄ and CF₃CF₃ therefore do not form anazeotropic mixture, they can be easily separated by any knowndistillation procedure.

The fluorination catalyst of step (1) is preferably a supported catalystor a bulk catalyst with trivalent chromium oxide as the main component.

In step (1), the molar ratio of the hydrogen fluoride gas and mixed gas(hydrogen fluoride/mixed gas containing hexafluoroethane andchlorotrifluoromethane) is preferably in the range of 0.05-10, and theconcentration of the chlorotrifluoromethane in the mixed gas of step (1)is preferably no greater than 0.1 vol %.

The mixed gas of step (1) is preferably obtained by reactingdichlorotetrafluoroethane and/or chloropentafluoroethane with hydrogenfluoride in a gas phase in the presence of a fluorination catalyst.Alternatively, the mixed gas of step (1) may be obtained by reaction oftetrafluoroethane and/or pentafluoroethane with fluorine gas.

The method of removing the acid component consisting of the hydrochloricacid produced by the reaction of formula (1) and the excess hydrogenfluoride may be, for example, a method of contacting with a purifyingagent or a method of contacting with water or an aqueous alkalisolution. The gas composed mainly of CF₃CF₃ after the acid component hasbeen removed may be dehydrated using a dehydrating agent such as zeoliteand subjected to a distillation procedure to cut the low boiling pointfraction, in order to obtain high-purity CF₃CF₃.

A hexafluoroethane production process according to a second aspect ofthe invention will now be explained in detail.

This process is a process for production of hexafluoroethane whichcomprises a step (1) in which pentafluoroethane containing chlorinecompounds with 1-3 carbon atoms is reacted with hydrogen in a gas phasein the presence of a hydrogenation catalyst at 150-400° C., forhydrogenation of the chlorine compounds, and a step (2) in which theproduct of step (1) is reacted with fluorine in a gas phase in thepresence of a diluent gas to produce hexafluoroethane.

As mentioned above, the pentafluoroethane used for this process isusually produced by fluorination of perchloroethylene (CCl₂═CCl₂) or itsfluorinated product with hydrogen fluoride (HF), and thepentafluoroethane will contain, as starting material-derived chlorinecompounds, chloromethane, chlorodifluoromethane,chloropentafluoroethane, dichlorotetrafluoroethane,chlorotetrafluoroethane, chlorotrifluoroethane, chlorotrifluoroethyleneand the like. A known distillation procedure may be employed for highpurification of the pentafluoroethane containing these compounds, butbecause the chlorine compounds and pentafluoroethane form azeotropicmixtures or azeotrope-like mixtures, their separation and purificationis extremely difficult, requiring an increased number of distillationsteps and distillation towers and thereby creating a problem, ineconomic terms, due to the added equipment and energy costs.

This process begins with step (1) in which the chlorine compoundscontaining chlorine atoms and included as impurities in thepentafluoroethane are first reacted with hydrogen in a gas phase in thepresence of a hydrogenation catalyst at 150-400° C. for hydrogenation,to convert them to hydrofluorocarbons (HFCs) and the like. For example,using hydrogen for hydrogenation of the chloropentafluoroethane(CF₃CClF₂) and chlorotetrafluoroethane (CF₃CHClF) included as impuritiesin the pentafluoroethane takes place the reactions represented byformulas (2) and (3) below.CF₃CClF₂+H₂→CF₃CHF₂+HCl  (2)CF₃CHClF+H₂→CF₃CH₂F+HCl  (3)

The products are hydrofluorocarbons containing no chlorine atoms, withhydrochloric acid as a by-product.

Compounds which are converted to hydrofluorocarbons by the abovehydrogenation reaction include the aforementioned chloromethane,chlorodifluoromethane, chlorotrifluoromethane, chloropentafluoroethane,dichlorotetrafluoroethane, chlorotetrafluoroethane,chlorotrifluoroethane, chlorotrifluoroethylene, etc., which are normallyincluded in pentafluoroethane in a total amount of a few thousand volppmor greater. When pentafluoroethane containing these compounds is reacteddirectly with fluorine gas, the methane-based compounds are convertedprimarily to chlorotrifluoromethane while the ethane-based compounds areconverted primarily to chloropentafluoroethane, and therefore thehexafluoroethane obtained after the reaction containschlorotrifluoromethane and chloropentafluoroethane as the majorimpurities.

The chloropentafluoroethane is virtually non-reactive with fluorine gasat low temperature. Investigation by the present inventors, however, hasindicated that when, for example, the reaction temperature is 400° C.and the chloropentafluoroethane concentration in pentafluoroethane is nogreater than about 800 volppm, the chloropentafluoroethane is cleaved,resulting in a chlorotrifluoromethane amount of no greater than 1volppm, but when the chloropentafluoroethane concentration is greaterthan about 2000 volppm, the chlorotrifluoromethane content reaches about2 volppm.

Since chlorotrifluoromethane forms an azeotropic mixture withhexafluoroethane, removal of this compound is very difficult bydistillation or adsorption procedures even when the concentration islow. Consequently, it is preferred not only to remove, from thepentafluoroethane starting material, the compounds which producechlorotrifluoromethane by reaction with fluorine gas, but also to reducethe chloropentafluoroethane content to as low a concentration aspossible.

The chlorine compounds included in the pentafluoroethane used for thisprocess are present at preferably no greater than 1 vol %, morepreferably no greater than 0.5 vol % and even more preferably no greaterthan 0.3 vol %. If the chlorine compound content exceeds 1 vol %, thereaction temperature must be increased, possibly resulting in a shorterlife of the hydrogenation catalyst.

As hydrogenation catalysts for step (1) there are preferred catalystscontaining at least one element from among platinum metals such asplatinum, palladium, rhodium, iridium, ruthenium and osmium, and thesemetals or their metal oxides or salts may be used as the startingmaterials. Carriers which may be used for these catalysts include activecarbon, alumina and fluorinated alumina, and the elementcomponent-supporting rate is preferably at least 0.02 wt % in order toefficiently promote the intended reaction. The hydrogenation catalystmay be prepared, for example, by dissolving the metal salt in an aqueoussolvent such as water, methanol or acetone, immersing the aforementionedcarrier in the solution for adsorption of the necessary element,distilling off the solvent, and accomplishing heat reduction treatmentwith hydrogen or the like.

The reaction temperature for the hydrogenation reaction of step (1) is150-400° C., and preferably 200-300° C. If the reaction temperature ishigher than 400° C. the catalyst life will tend to be shortened, oftenpromoting excess reaction. Excessive promotion of the hydrogenationreaction is undesirable because of production of 1,1,1-trifluoroethane,etc. Reaction control is difficult in step (2) in which the organiccompound and fluorine gas participate in direct fluorination reaction,because a very large amount of reaction heat is generated and, forexample, a greater reaction heat results with a larger number of C—Hbonds substituted with C—F bonds in the organic compound substrate, thustending to produce partial heat generation (hot spots). Consequently, itis preferred to use pentafluoroethane containing minimalhydrofluorocarbons (HFCs), and especially 1,1,1-trifluoroethane whichcontains numerous C—H bonds. On the other hand, a reaction temperatureof lower than 150° C. will tend to inhibit the intended reaction.

The molar ratio of hydrogen and the mixed gas (hydrogen/chlorinecompound and pentafluoroethane mixed gas) for the hydrogenation reactionof step (1) is preferably in the range of 1-20, and more preferably inthe range of 2-10. The reaction pressure is preferably in a range fromatmospheric pressure to 1.5 MPa. A reaction pressure exceeding 1.5 MPacan be problematic since it necessitates pressure-resistant equipment.

In this process, step (1) is carried out under the reaction conditionsdescribed above, and the reaction product will contain, along withpentafluoroethane, also hydrofluorocarbons containing no chlorine atoms,and trace amounts of unreacted chlorine compounds and acid componentby-products such as hydrochloric acid, which are preferably removed.

The method of removing the acid components may be, for example, a methodof contacting with a purifying agent or a method of contacting withwater or an aqueous alkali solution. The gas from which the acidcomponents have been removed is preferably dehydrated using adehydrating agent such as zeolite and then distilled before the directfluorination step to obtain purified pentafluoroethane, and theunreacted hydrogen is preferably separated out. Hydrogen is preferablynot included in the direct fluorination reaction of step (2), because itmay react violently with fluorine gas.

The chlorine compounds in the mixed gas obtained by step (1) arepreferably present in a total amount of no greater than 0.05 vol %, and1,1,1-trifluoroethane is preferably present at no greater than 0.2 vol%.

The following explanation concerns step (2) in which the gas containingthe pentafluoroethane obtained by step (1) as the major component isreacted with fluorine gas.

Step (2) is carried out in the presence of a diluent gas, in order toset the concentration of the mixed gas with pentafluoroethane as themajor component to below the explosive range. Specifically, theconcentration of the pentafluoroethane at the reactor inlet ispreferably no greater than about 6 mole percent. The diluent gas usedcontains at least one selected from the group consisting oftetrafluoromethane, hexafluoroethane, octafluoropropane and hydrogenfluoride, and preferably a diluent gas rich in hydrogen fluoride isused. A diluent gas rich in hydrogen fluoride is a diluent gascontaining at least 50 mole percent hydrogen fluoride.

The amount of fluorine gas used may be in the range of 0.5-2.0 andpreferably in the range of 0.9-1.3, in terms of the molar ratio withrespect to the mixed gas containing pentafluoroethane as the majorcomponent (F₂/hydrogenated compound-containing pentafluoroethane). Thereaction temperature is preferably 250-500° C., and more preferably350-450° C. If the reaction temperature is higher than 500° C., thetarget product, hexafluoroethane, may be cleaved, producingtetrafluoromethane (CF₄). If the reaction temperature is below 250° C.,the reaction rate may be slowed.

The method of purifying the effluent gas from step (2) is notparticularly restricted, but preferably the residual unreacted fluorinegas is removed first. For example, a hydrofluorocarbon such astrifluoromethane may be added to react with and remove the excessfluorine gas. This is preferably followed by distillation, for example,distillation of the hydrogen fluoride and organic matter first. Theseparated hydrogen fluoride may be reused as diluent gas for the directfluorination reaction of step (2), or it may be utilized for a differentpurpose.

The composition of the gas containing the separated organic matter willdiffer considerably depending on the diluent gas used for the reaction,and for example, when a hydrogen fluoride-rich gas or hexafluoroethaneitself is used as the diluent gas, the gas obtained by the reaction willcontain hexafluoroethane as the major component. When tetrafluoromethaneor octafluoropropane is used as the diluent gas, purification isachieved by further distillation, but in either case, high-purityhexafluoroethane can be obtained by repeated distillation of the mixedgas which is obtained.

The distillation and purification of the mixed gas obtained by thereaction will depend on the compositional ratio, but as an example, lowboiling point components such as inert gases and tetrafluoromethane areextracted from the top of a first distillation tower, and a mixed gascomposed mainly of hexafluoroethane is extracted from the bottom. Themixed gas extracted from the bottom is then introduced into a seconddistillation tower and the low boiling point components such as inertgases and trifluoromethane are extracted from the top of the seconddistillation tower, after which the mixed gas composed mainly ofhexafluoroethane which is extracted from the bottom is introduced into athird distillation tower and high-purity hexafluoroethane is extractedfrom the top of the tower, thereby accomplishing purification.

The process of the invention described above may be used to obtainhexafluoroethane with a purity of 99.9997 vol % or greater. Thechlorotrifluoromethane included as an impurity is therefore less than 1volppm. CF₃CF₃ with a purity of 99.9997 vol % or greater can be analyzedby gas chromatography (GC) methods such as TCD, FID (both includingprecut methods), ECD or with a gas chromatography-mass spectrometer(GC-MS).

The uses of high-purity hexafluoroethane obtained by the process of theinvention will now be explained. High-purity CF₃CF₃, or mixed gases ofhigh-purity CF₃CF₃ with inert gases such as He, N₂ and Ar or gases suchas O₂ and NF₃ (for the purpose of the invention, these will becollectively referred to as “hexafluoroethane products”) may be used asetching gases for etching steps in semiconductor device manufacturingprocesses or as cleaning gases for cleaning steps in semiconductordevice manufacturing processes. In manufacturing processes forsemiconductor devices such as LSIs and TFTs, thin-films and thick-filmsare formed using CVD, sputtering, vapor deposition and the like, andthese are etched to form circuit patterns. The thin-film and thick-filmforming apparatuses must be cleaned in order to remove unwantedaccumulated matter in the apparatus inner walls, jigs, etc. Productionof unwanted accumulated matter can result in particle generation,requiring constant removal in order to produce satisfactory films.

Etching methods using CF₃CF₃ may be carried out under various dryetching conditions such as plasma etching, microwave etching or thelike, in which case the CF₃CF₃ may be used as a mixture with inert gasessuch as He, N₂ or Ar or gases such as HCl, O₂, H₂, F₂, NF₃ or the likein appropriate proportions.

The present invention will now be explained in greater detail by way ofexamples, with the understanding that the invention is in no way limitedby the examples.

Pentafluoroethane Production Example 1 (Starting Material Example 1)

Tetrachloroethylene (CCl₂═CCl₂) was contacted with Molecular Sieve 4A(product of Union Showa Co., Ltd.) to remove the stabilizer and moisturein the tetrachloroethylene, and was then reacted with hydrogen fluoride(HF) in the presence of a chromium-based fluorination catalyst (firstreaction: Reaction pressure of 0.4 MPa, reaction temperature of 320° C.,HF/tetrachloroethylene=8 (molar ratio)). Next, the primarilydichlorotrifluoroethane (CF₃CHCl₂) and chlorotetrafluoroethane(CF₃CHClF) product obtained by the first reaction was reacted withhydrogen fluoride (second reaction: Reaction pressure of 0.45 MPa,reaction temperature of 330° C., HF/(CF₃CHCl₂+CF₃CHClF)=6 (molarratio)). After completion of the second reaction, the acid component wasremoved by a known method, and distillation purification was performedto obtain a distillate containing pentafluoroethane as the majorcomponent. The distillate was analyzed by gas chromatography andidentified as pentafluoroethane having the composition shown in Table 1.

TABLE 1 Compound Purity (vol %) CF₃CHF₂ 99.7003 CH₃Cl 0.0014 CHClF₂0.0009 CHF₃ 0.0127 CF₃CClF₂ 0.2819 CF₃CHClF 0.0008 CF₃CCl₂F 0.0009 Other0.0011

Starting Material Hexafluoroethane Production Example 1 (StartingMaterial Example 2)

Nitrogen gas was introduced into an Inconel 600 reactor with an innerdiameter of 20.6 mm and a length of 500 mm (electric heater type:passivation treated with fluorine gas at a temperature of 500° C.)through two gas inlets at a total flow rate of 30 NL/hr, and thetemperature in the reactor was kept at 380° C. Next, hydrogen fluoridewas introduced through the two gas inlets at a total flow rate of 50NL/hr, and pentafluoroethane (Starting Material Example 1) obtained inPentafluoroethane Production Example 1 was introduced through one of thegas inlets at a flow rate of 3.6 NL/hr. Fluorine gas was introducedthrough the other gas inlet at a flow rate of 3.9 NL/hr for fluorinationreaction. The outlet gas from the reactor was contacted with an aqueouspotassium hydroxide solution and an aqueous potassium iodide solution,and the hydrogen fluoride and unreacted fluorine gas in the outlet gaswere removed. After contact with a dehydrating agent for drying, thedried gas was cooled and collected, and distilled for purification. Thepurified hexafluoroethane was analyzed by gas chromatography, giving theresults shown in Table 2.

TABLE 2 Compound Purity (vol %) CF₃CF₃ 99.9972 CF₄ <0.0001 CClF₃ 0.0025CF₃CHF₂ 0.0001 Other 0.0001

Catalyst Production Example 1 (Catalyst Example 1)

After placing 0.6 L of pure water in a 10 L vessel and stirring, asolution of 452 g of Cr(NO₃)₃.9H₂O and 42 g of In(NO₃)₃.nH₂O (where n isapproximately 5) in 1.2 L of purified water and 0.31 L of 28% aqueousammonia water were added dropwise thereto, to a reaction solution pH inthe range of 7.5-8.5 over a period of about 1 hour while controlling theflow rate of the two aqueous solutions. The obtained slurry wasfiltered, and the filtered solid was thoroughly washed with purifiedwater and then dried at 120° C. for 12 hours. After pulverizing thedried solid, it was mixed with graphite and shaped into pellets using atablet molding machine. The pellets were calcined for 4 hours under anitrogen stream at 400° C. to prepare a catalyst precursor. The catalystprecursor was filled into an Inconel reactor and subjected tofluorination treatment (catalyst activation) under a nitrogen-dilutedhydrogen fluoride stream at 350° C. and ordinary pressure. It was thensubjected to fluorination treatment (catalyst activation) at 400° C.under a 100% hydrogen fluoride stream and then under a nitrogen-dilutedhydrogen fluoride stream, to prepare a catalyst (Catalyst Example 1).

EXAMPLE 1

A 150 ml portion of the catalyst (Catalyst Example 1) obtained inCatalyst Production Example 1 was filled into an Inconel 600 reactorwith an inner diameter of 1 inch and a length of 1 m and the temperaturewas kept at 280° C. while circulating nitrogen gas. Hydrogen fluoridewas then supplied at 1.5 NL/hr, and starting material hexafluoroethanehaving the composition shown in Table 2 (Starting Material Example 2)was supplied at 3.5 NL/hr. The supply of nitrogen gas was then suspendedand reaction was initiated. After 3 hours, the outlet gas from thereactor was washed with an aqueous potassium hydroxide solution toremove the acid component, and the gas composition was analyzed by gaschromatography and identified as hexafluoroethane having the compositionshown in Table 3.

TABLE 3 Compound Purity (vol %) CF₃CF₃ 99.9972 CF₄ 0.0025 CClF₃ <0.0001CF₃CHF₂ 0.0001 Other <0.0001

As shown in Table 3, the chlorotrifluoromethane contained inhexafluoroethane can be converted to tetrafluoromethane.

The above reaction was then carried out and the acid component-removedgas was contacted with a dehydrating agent, dried, cooled and collected,and then distilled by a known method for purification. Thehexafluoroethane obtained by the distillation was analyzed by gaschromatography, giving the results shown in Table 4.

TABLE 4 Compound Purity CF₃CF₃ ≧99.9997 vol % CF₄ <0.5 volppm CClF₃ <0.5volppm CF₃CHF₂ <0.5 volppm Other <1.0 volppm

Based on the analysis results shown in Table 4, the purity of thehexafluoroethane was at least 99.9997 vol %, and thechlorotrifluoromethane content was less than 0.5 volppm. The impuritieswere analyzed by gas chromatography using TCD, FID, ECD and GC-MS.

EXAMPLE 2

A 100 ml portion of the catalyst (Catalyst Example 1) obtained inCatalyst Production Example 1 was filled into an Inconel 600 reactorwith an inner diameter of 1 inch and a length of 1 m and the temperaturewas kept at 400° C. while circulating nitrogen gas. Hydrogen fluoridewas then supplied at 10 NL/hr, and starting material hexafluoroethanehaving the composition shown in Table 2 (Starting Material Example 2)was supplied at 10 NL/hr. The supply of nitrogen gas was then suspendedand reaction was initiated. After 3 hours, the outlet gas from thereactor was treated for removal of the acid component in the same manneras Example 1, and analysis was performed by gas chromatography. Theresults confirmed the product to be hexafluoroethane having thecomposition shown in Table 5.

TABLE 5 Compound Purity (vol %) CF₃CF₃ 99.9972 CF₄  0.0026 CClF₃ NDOther  0.0002

In Table 5, ND means “not detectable” by GC-MS, i.e. a value of lessthan 0.1 volppm. The CClF₃ content was therefore less than 0.1 volppm.

The product gas dehydrated in the same manner as Example 1 was cooledand collected, and distilled under the same conditions, and theresulting hexafluoroethane was analyzed by gas chromatography, givingthe results shown in Table 6.

TABLE 6 Compound Purity CF₃CF₃ >99.9998 vol % CF₄ <0.5 volppm CClF₃ <0.1volppm Other <1.0 volppm

As clearly shown by the analysis results in Table 6, the CClF₃ contentwas less than 0.1 volppm, and therefore high-purity hexafluoroethane hadbeen obtained.

Pentafluoroethane Production Example 2 (Starting Material Example 3)

Tetrachloroethylene (CCl₂═CCl₂) was contacted with Molecular Sieve 4A(product of Union Showa K.K.) to remove the stabilizer and moisture inthe tetrachloroethylene, and was then reacted with hydrogen fluoride(HF) in the presence of a chromium-based fluorination catalyst (firstreaction: Reaction pressure of 0.4 MPa, reaction temperature of 300° C.,HF/tetrachloroethylene=8 (molar ratio)). Next, the mixed gas containingprimarily dichlorotrifluoroethane (CF₃CHCl₂) and chlorotetrafluoroethane(CF₃CHClF) obtained by the first reaction was reacted with hydrogenfluoride (second reaction: Reaction pressure of 0.4 MPa, reactiontemperature of 330° C., HF/(CF₃CHCl₂+CF₃CHClF)=6 (molar ratio)). Aftercompletion of the second reaction, the acid component was removed by aknown method, and distillation purification was performed to obtain adistillate containing mostly pentafluoroethane as the major component.The distillate was analyzed by gas chromatography and identified as amixed gas having the composition shown in Table 7 (Starting MaterialExample 3).

TABLE 7 Compound Purity (vol %) CF₃CHF₂ 99.4290 CH₃Cl 0.0011 CHClF₂0.0008 CHF₃ 0.0218 CClF₃ 0.0006 CF₃CClF₂ 0.5439 CF₃CHClF 0.0007 CF₃CCl₂F0.0011 CF₃CH₂Cl 0.0003 Other 0.0007

Pentafluoroethane Production Example 3 (Starting Material Example 4)

The mixed gas containing pentafluoroethane as the major component whichwas obtained by the process described above (Starting Material Example3) was distilled by a known method. The distillate was analyzed by gaschromatography and identified as a mixed gas having the compositionshown in Table 8 (Starting Material Example 4).

TABLE 8 Compound Purity (vol %) CF₃CHF₂ 99.7984 CHClF₂ 0.0003 CHF₃0.0024 CF₃CClF₂ 0.1989

Catalyst Production Example 2 (Catalyst Example 2)

Sodium palladium chloride was dissolved in water, and a 1.6 mmφspherical alumina support was immersed therein for adsorption of thepalladium salt, after which the solvent was distilled off at atemperature of 100° C., calcining was conducted in air at 300° C., andhydrogen reduction was accomplished at 350° C. The supporting rate ofthe obtained palladium catalyst was 2.0%.

Catalyst Production Example 3 (Catalyst Example 3)

Platinic chloride was dissolved in water, and a 1.6 mmφ sphericalalumina support was immersed therein for adsorption of the platinumsalt, after which the solvent was distilled off at a temperature of 100°C., calcining was conducted in air at 300° C., and hydrogen reductionwas accomplished at 350° C. The supporting rate of the obtained platinumcatalyst was 2.0%.

EXAMPLE 3

A 100 ml portion of the catalyst (Catalyst Example 2) obtained inCatalyst Production Example 2 was filled into an Inconel 600 reactorwith an inner diameter of 1 inch and a length of 1 m and the temperaturewas kept at 280° C. while circulating nitrogen gas. Hydrogen was thensupplied at a flow rate of 0.36 NL/hr, the mixed gas having thecomposition shown in Table 7 (Starting Material Example 3) was suppliedat 8.33 NL/hr, and then the supply of nitrogen gas was suspended and areaction (step (1)) was initiated. After 2 hours, the outlet gas fromthe reactor was washed with an aqueous solution of potassium hydroxidefor removal of the acid component, and then analysis of the gascomposition by gas chromatography identified it as a mixed gas composedmainly of pentafluoroethane having the composition shown in Table 9.

TABLE 9 Compound Purity (vol %) CF₃CHF₂ 99.9177 CH₄ 0.0013 CH₂F₂ 0.0009CHF₃ 0.0226 CF₃CH₂F 0.0126 CF₃CH₃ 0.0386 CF₃CClF₂ 0.0058 Other 0.0005

The mixed gas composed mainly of pentafluoroethane having thecomposition shown in Table 9, which was obtained by the processdescribed above, was distilled by a known method, and after removing thelow boiling point components such as the inert gas and hydrogen gas, adirect fluorination reaction (step (2)) was conducted with fluorine gas.

Nitrogen gas was introduced into an Inconel 600 reactor with an innerdiameter of 20.6 mmφ and a length of 500 mm (electric heater typereactor: passivation treated with fluorine gas at a temperature of 500°C.) through two gas inlets at a total flow rate of 30 NL/hr, and thetemperature in the reactor was kept at 420° C. Next, hydrogen fluoridewas introduced through the two gas inlets at a total flow rate of 50NL/hr, and the above-mentioned mixed gas composed mainly ofpentafluoroethane from which the low boiling point components had beenremoved was introduced through one of the gas inlets at a flow rate of3.5 NL/hr. Fluorine gas was introduced through the other gas inlet at aflow rate of 3.85 NL/hr and reacted therewith. After 3 hours, the outletgas from the reactor was contacted with an aqueous potassium hydroxidesolution and an aqueous potassium iodide solution, and the hydrogenfluoride and unreacted fluorine gas were removed. After contact with adehydrating agent for drying, the dried gas composition was analyzed bygas chromatography. The analysis results are shown in Table 10.

TABLE 10 Compound Purity (vol %) CF₃CF₃ 99.9541 CF₄ 0.0388 CHF₃ 0.0012CF₃CHF₂ <0.0001 CF₃CH₂F trace CF₃CClF₂ 0.0058

The dried gas composed mainly of hexafluoroethane was cooled andcollected, and purified by distillation. The purified gas was analyzedby gas chromatography using TCD, FID, ECD and GC-MS, and the resultsshown in Table 11 were obtained.

TABLE 11 Compound Purity volppm CF₃CHF₂   <0.5 volppm CF₃CH₂F   <0.5volppm CF₄   <0.4 volppm SF₆   <0.4 volppm CF₃CClF₂   <0.1 volppmCF₃CF₃ >99.9998 vol%

As clearly seen by the analysis results in Table 11, thehexafluoroethane contained virtually no impurities, thus demonstratingthat high-purity hexafluoroethane had been obtained, with a purity of atleast 99.9997 vol %.

EXAMPLE 4

A reaction (step (1)) was carried out with the same conditions andprocedure as Example 3, except that 100 ml of catalyst (Catalyst Example3) was filled in and the mixed gas of Starting Material Example 4 wasused. The reactor outlet gas was treated in the same manner and thenanalyzed, giving the results shown in Table 12.

TABLE 12 Compound Purity (vol %) CF₃CHF₂ 99.9692 CH₄ 0.0002 CH₂F₂ 0.0002CF₃CH₂F 0.0056 CF₃CH₃ 0.0215 CF₃CClF₂ 0.0012 CHF₃ 0.0021

The mixed gas composed mainly of pentafluoroethane having thecomposition shown in Table 12, which was obtained by the processdescribed above, was distilled by a known method, and after removing thelow boiling point components such as the inert gas and hydrogen gas, adirect fluorination reaction (step (2)) was conducted with fluorine gasusing the same conditions and procedure as Example 3. The gas obtainedby treating the reactor outlet gas according to the same process asExample 3 was analyzed by gas chromatography, giving the results shownin Table 13.

TABLE 13 Compound Purity (vol %) CF₃CF₃ 99.9680 CF₄ 0.0314 CHF₃ 0.0002CF₃CHF₂ <0.0001 CF₃CH₂F <0.0001 CF₃CH₃ <0.0001 CF₃CClF₂ <0.0001

The mixed gas composed mainly of hexafluoroethane was cooled andcollected, and then purified by distillation. Analysis of the purifiedgas indicated a hexafluoroethane purity of at least 99.9998 vol %, achlorine compound impurity content of less than 1 volppm, and a startingmaterial pentafluoroethane content of less than 1 volppm.

COMPARATIVE EXAMPLE 1

A reaction (step (1)) was conducted using the same conditions andprocedure as Example 3, except for a reaction temperature of 430° C.,and the product was analyzed. The results are shown in Table 14.

TABLE 14 Compound Purity (vol %) CF₃CHF₂ 99.5373 CH₄ 0.0134 C₂H₆ 0.0098CH₂F₂ 0.0002 CHF₃ 0.0189 CF₃CH₂F 0.0043 CF₃CH₃ 0.4150 CF₃CClF₂ 0.0044Other 0.0011

As clearly seen from the analysis results in Table 14, when step (1) wasconducted at a reaction temperature of higher than 400° C., excesshydrogenation reaction was promoted, thereby notably increasing theamount of 1,1,1-trifluoroethane product. Methane and ethane were alsoproduced, and deterioration of the catalyst was observed.

COMPARATIVE EXAMPLE 2

A reaction (step (1)) was conducted with the same conditions andprocedure as Example 3, except for a reaction temperature of 130° C.,and the product was analyzed. When step (1) was conducted at a reactiontemperature of lower than 150° C., hydrogenation reaction of thechlorine compounds was almost completely inhibited, resulting in achloropentafluoroethane conversion rate of about 19%.

COMPARATIVE EXAMPLE 3

A direct fluorination reaction (step (2)) was conducted using the sameconditions and procedure as Example 3, except for using the mixed gas ofStarting Material Example 3, and after removing the hydrogen fluorideand unreacted fluorine gas contained in the reactor outlet gas, analysiswas performed by gas chromatography. The results are shown in Table 15.

TABLE 15 Compound Purity (vol %) CF₃CF₃ 99.4210 CF₄ 0.0308 CClF₃ 0.0046CF₃CHF₂ 0.0002 CF₃CClF₂ 0.5419 CF₃CCl₂F 0.0009 Other 0.0006

As clearly seen in Table 15, direct fluorination reaction usingpentafluoroethane with a high chlorine compound content producedchlorotrifluoromethane, which is difficult to separate.

INDUSTRIAL APPLICABILITY

According to the procedure of the present invention, it is possible toobtain high-purity CF₃CF₃ and the obtained CF₃CF₃ is suitable for use asan etching gas or cleaning gas for semiconductor device manufacturingsteps.

1. A process for production of hexafluoroethane which comprises a stepin which a mixed gas containing hexafluoroethane andchlorotrifluoromethane is reacted with hydrogen fluoride in a gas phasein the presence of a fluorination catalyst at 200-450° C., forfluorination of said chlorotrifluoromethane.
 2. A process according toclaim 1, wherein the fluorination catalyst is a supported catalyst or abulk catalyst with trivalent chromium oxide as the major component.
 3. Aprocess according to claim 1, wherein the molar ratio of the hydrogenfluoride and said mixed gas (hydrogen fluoride/mixed gas containinghexafluoroethane and chlorotrifluoromethane) is in the range of 0.05-10.4. A process according to any one of claims 1 to 3, wherein theconcentration of the chlorotrifluoromethane in said mixed gas is nogreater than 0.1 vol %.
 5. A process according to any one of claims 1 to3, wherein said mixed gas is obtained by reactingdichlorotetrafluoroethane and/or chloropentafluoroethane with hydrogenfluoride in a gas phase in the presence of a fluorination catalyst.
 6. Aprocess according to any one of claims 1 to 3, wherein said mixed gas isobtained by reacting tetrafluoroethane and/or pentafluoroethane withfluorine gas.
 7. A process for production of hexafluoroethane whichcomprises the following steps (1) and (2): (1) A step in which a mixedgas containing hexafluoroethane and chlorotrifluoromethane is reactedwith hydrogen fluoride in a gas phase in the presence of a fluorinationcatalyst at 200-450° C. to fluorinate said chlorotrifluoromethane andconvert it to tetrafluoromethane, and (2) A step in which the mixed gascontaining hexafluoroethane and tetrafluoromethane obtained in step (1)is distilled to obtain purified hexafluoroethane.
 8. A process accordingto claim 7, wherein the fluorination catalyst of step (1) is a supportedcatalyst or a bulk catalyst with trivalent chromium oxide as the majorcomponent.
 9. A process according to claim 7, wherein in step (1), themolar ratio of the hydrogen fluoride and said mixed gas (hydrogenfluoride/mixed gas containing hexafluoroethane andchlorotrifluoromethane) is in the range of 0.05-10.
 10. A processaccording to any one of claims 7 to 9, wherein the concentration of thechlorotrifluoromethane in said mixed gas of step (1) is no greater than0.1 vol %.
 11. A process according to any one of claims 7 to 9, whereinsaid mixed gas of step (1) is obtained by reactingdichlorotetrafluoroethane and/or chloropentafluoroethane with hydrogenfluoride in a gas phase in the presence of a fluorination catalyst. 12.A process according to any one of claims 7 to 9, wherein said mixed gasof step (1) is obtained by reacting tetrafluoroethane and/orpentafluoroethane with fluorine gas.